Item 35 - James David Forbes to William Whewell

4 pp

Edinburgh - Thanks WW for his book on the Doctrine of Limits [The Doctrine of Limits with its Applications Namely Conic Sections, the First Three Sections of Newton, the Differential Calculus, 1838], and congratulates him on his award of the Royal Society Medal for his work on tides. JDF has had news that John Herschel is soon to return [from the Cape in South Africa]. JDF has been working hard on the subject of Heat: 'I have got an unexceptionable method of measuring Indices of Refraction of heat (which you are probably aware has hitherto been unsuccessfully attempted - numerically I mean, -) by observing the critical angle of Total Reflection, a method which may probably one day be so far improved as to give the means of analyzing a ray of heat into its component fasciculi between definite limits of refrangibility. In the next place I have worked out numerically the law of Depolarization which when the length of a wave is given will give the retardation and vice versa'. Macedoni Melloni began his long paper on depolarization by trying to 'pick holes' in JDF's experiments 'very ungraciously and substitute his own for them', but 'he ends in confirming all my results and adding almost nothing of his own, with a single exception viz. he finds all kinds of heat equally polarizable; I find it less so as the Temp. is lower - obviously a point of great importance for Theory'. His discrepancy with Melloni concerns the experimental set-up: 'Melloni instead of using my excessively thin polarizing bundles of mica split by heat, employs the old piles of distinct mica plates as I first used them. the fact is the heat in being polarized passes through so much mica, that it loses its distinctive character and whatever be the source consists at last only of that kind of heat which mica is capable of transmitting'. Conversely, JDF's extra thin mica piles 'suffers heat from different sources to pass with almost equal readiness. so there is an end of the puzzle'.